In recent years, in order to increase the transmission capacity of an optical transmission system, an optical transmission system that has a high bit rate ranging to 100 Gbit/s has been researched and developed.
As an optical modulation method considered most promising for transmitting an optical signal that has such a high bit rate, a multilevel phase modulation method is being actively researched.
For example, a quadrature phase shift keying (QPSK) method or a differential quadrature phase shift keying (DQPSK) method is cited. Using such a method, a signal that has two bits can be transmitted. Accordingly, compared with a binary phase shift keying (BPSK) method or a differential binary phase shift keying (DBPSK) method, the transmission capacity of an optical transmission system can be doubled, or the bit rate thereof for obtaining the same transmission capacity can be decreased by half.
Recently, as a modulation method that can further increase the transmission capacity of an optical transmission system, an eight phase shift keying (8PSK) method or a differential eight phase shift keying (D8PSK) method has been researched and developed. Using such a method, a signal that has three bits can be transmitted. Accordingly, compared with the BPSK method or the DBPSK method, the transmission capacity of an optical transmission system can be tripled, or the bit rate thereof for obtaining the same transmission capacity can be reduced to ⅓.
A coherent optical receiver that includes a 45-degree hybrid is necessary for demodulating signal light modulated by such an 8PSK system or a D8PSK system as just described. Here, the 45-degree hybrid exhibits output forms that have different branching ratios depending upon the phase modulation states of 8PSK signal light or D8PSK signal light, respectively, and is the most important component of the coherent optical receiver.
For example, as illustrated in FIG. 21, as the 45-degree hybrid, there is a 45-degree hybrid that includes two 1:4 couplers (6 dB couplers) 100 and 101 and four 2:2 couplers (3 dB couplers) 102 to 105.
In addition, in FIG. 21, “I” indicates an in-phase relationship (namely, a phase relationship located on an I axis (0-degree axis)) in a phase relationship diagram. In addition, in FIG. 21, “Q” indicates a quadrature relationship (namely, a phase relationship displaced by 90 degrees with respect to the I axis [a phase relationship located on a Q axis (90-degree axis)]) in the phase relationship diagram. In addition, in FIG. 21, “I+Q” indicates a 45-degree relationship (namely, a phase relationship displaced by 45 degrees with respect to the I axis) in the phase relationship diagram. In addition, in FIG. 21, “I−Q” indicates a 135-degree relationship (namely, a phase relationship displaced by 135 degrees with respect to the I axis) in the phase relationship diagram.
In the 45-degree hybrid as illustrated in FIG. 21, 8PSK signal light is input to the input channel of one 1:4 coupler 100; and LO light is input to the input channel of the other 1:4 coupler 101.
In addition, a pair of optical signals (indicated by “I” in FIG. 21) that indicate an in-phase relationship (In-phase) is output from output channels positioned on the first from individual sides (upper sides in FIG. 21) of the two 1:4 couplers 100 and 101.
In addition, a pair of optical signals (indicated by “Q” in FIG. 21) that indicate the quadrature phase (Quadrature) is output from output channels positioned on the second from the individual sides (the upper sides in FIG. 21) of the two 1:4 couplers 100 and 101.
In addition, a pair of optical signals (indicated by “I+Q” in FIG. 21) that indicate the 45-degree phase relationship is output from output channels positioned on the third from the individual sides (the upper sides in FIG. 21) of the two 1:4 couplers 100 and 101.
In addition, a pair of optical signals (indicated by “I−Q” in FIG. 21) that indicate the 135-degree phase relationship is output from output channels positioned on the fourth from the individual sides (the upper sides in FIG. 21) of the two 1:4 couplers 100 and 101.